"The Commissioners of Bridgeville may, by ordinance, provide for a qualified voter, duly registered, who shall be unable to appear to cast a ballot at any annual election either because of being in the public service of the United States or of this State, or because of the nature of hts or her business or occupation, or because of illness or physical disability, or because of being unavoidably absent on the day of the election, or because of being absent while on vacation, to vote by absentee ballot."